The present invention relates generally to all channel television tuning systems and more particularly to a varactor diode tuning system having a time sequenced presettable counting system for determining the system tuning condition by counting the frequency of a tunable element such as a local heterodyne oscillator.
Tuning of broadcast frequencies with a tuned circuit employing a varactor diode in combination with an inductor is accomplished by varying the voltage across the diode to produce a change in its capacitance. The current and voltage characteristics of varactor diodes are well known. The capacitance exhibited varies as a function of the reverse bias voltage applied across their terminals. Since the diodes operate in the depletion mode, the larger the reverse bias, the smaller the capacitance exhibited. When used for the low VHF television frequencies, the bias voltage (tuning voltage) is small (high capacitance) and certain temporary operating conditions may give rise to an unstable operating environment for the oscillator, which may be manifested by very weak oscillator output or intermittent oscillatory operation. In either situation, the number of pulses counted by the counting means for a given tuning voltage may be substantially less than that actually generated, or supposed to be generated. With the large preset counts used for the low VHF bands, a temporary malfunction of this nature could produce an insufficient number of pulses for the counter resulting in an indication of tuning to a very high channel number (frequency) whereas, in fact, the opposite situation prevails. Under this condition the comparator would drive the tuning voltage in a direction opposite to that which is actually required, and the system coulld "lock up" and render the temporary malfunction permanent.
Specifically, in the above-mentioned Tanaka application the preset count is 85 for the low VHF bands and, for example, channel 2 would require counting 17 CU pulses from the modular scaler. If only 13 CU pulses were counted, the derived channel number would give an indication of the system being tuned to channel 98 and system response (driving the voltage even lower in an effort to tune to channel 2) could result in extinguishing oscillation.